40 MINERALOGY. 



68. Obtaining Metals from their Ores. Sometimes this 

 is a very simple process. For example, in the case of 

 bismuth, all that is necessary is to heat the pounded ore, 

 and the melted metal runs out. So, if you have a sul- 

 phuret of lead, or mercury, or antimony, heat will fully 

 decompose the compound, driving off the sulphur. But 

 sometimes the use of some other substance is required 

 for the decomposition of the ore. Thus, if we have an 

 oxyd of iron, we heat it with charcoal. Here the oxygen 

 quits the iron to unite with the carbon, forming carbonic 

 acid, which flies off, leaving the metallic iron. 



69. Gangues and Impurities. The rock in which an 

 ore is found is called the gangue. Much of this is separ- 

 ated from the ore in collecting it, and much more of it, 

 perhaps, by the process called washing, in which the ma- 

 terial, coarsely powdered, is* subjected to a current of 

 water, that washes away the lighter pieces, leaving those 

 which are rendered heavy by the presence of the metal. 

 The impurities which are often mingled with ores are 

 got rid of in various ways. Sometimes fluxes, so called, 

 are used for this purpose. This is done with most iron 

 ores. There is commonly mixed with these ores quartz 

 or clay ; and as quartz is pure silica, and silica consti- 

 tutes 75 per cent, of clay, the ore is reduced by strongly 

 heating it together with a substance which will form a 

 silicate, that is, a glass, with the silica ( 342, Part II.). 

 Such a substance is common limestone. 



Other processes will be noticed hereafter in the case 

 of particular ores. 



70. Iron. The most common of the ores of this metnl 

 are oxyds and sulphurets. Of the latter I have already 

 spoken in 50. Its ores are more abundant in the earth 

 than those of any other metal, because it is the most ex- 

 tensively and variously useful of all the metals. They 

 are the common coloring ingredients of rocks, and there- 

 fore of soils. Red and yellow are the most frequent col- 

 ors, but they color also a dull green, brown, and black. 



